Many approaches have been taken to measuring while drilling or logging while drilling. Most approaches have utilized pulsing or vibrating the drilling mud column within the drill pipe for signal transmission.
One known approach to transmitting signals from the well bore bottom to the earth's surface electrically is disclosed in U.S. Pat. No. 4,057,781. This U.S. Pat. No. 4,057,781, which discloses a system herein referred to as the "cascade" communication system, is herein specifically incorporated by reference for its background information.
A problem with subsurface telemetry has been that, in large part, batteries are required to operate the detection and transmission circuitry at the bottom of the well bore and also for relaying intermittant signals from places along the drilling string such as in the cascade system above referenced. Batteries are prone to become much less efficient and finally inoperative at temperatures in excess of about 300.degree. F. and, at present, no batteries are commercially available which are operable for any appreciable length of time in some hot, deep wells.
In some of the prior art, downhole electrical power generators have been provided which have performed satisfactorily so long as such generators remained in operable condition. A big problem encountered in the dowhole generators is keeping the mud out of the generator proper. Also, with some of the generators which have operated in an oil bath, the fluid seals separating the oil from the surrounding drilling mud have deteriorated rapidly due to the abrasive qualities of the drilling mud and, once the drilling mud has leaked into the generator, the generator usually becomes inoperative due to bearing failure and the like.
The impeller driven power generator as disclosed for use in the present invention is also an improvement over known power generators which have been employed for use within drill pipe. The present power generator is adapted to run continuously for many hours with no mechanical deterioration other than the normal wear and tear on its impeller blades.